The present invention relates to an optical isolator which is an optical communication system and an optical measuring instrument and which is used for preventing light rays emitted from a light source and made incident upon an optical system from being reflected back to the light source at the end face of the optical system.
When light rays emitted from a light source are to be transmitted through an optical system, part of the light rays are reflected at the end face of the optical system and returned back to the light source. For instance, in transmitting an optical signal through an optical fiber, a light beam emitted from a laser light source is projected onto the end face of the optical fiber through a lense and the majority thereof is transmitted through the optical fiber as transmitted light beam, but a part thereof is surface-reflected at the end faces of the lense and the optical fiber and returned back to the laser light source. The reflected light is again reflected at the surface of the laser light source to thus make noises.
To eliminate such noises, there has been used an optical isolator. The optical isolator comprises, as shown in FIG. 1, a polarizer 1, a Farady rotator 3 and an analyzer 2 which are arranged in this order. The polarizer 1 has a polarization direction indicated by an arrow z. The Farady rotator 3 is positioned in a magnetic field S.fwdarw.N and rotates the plane of polarization of the transmitted light at an angle of 45.degree. clockwise as viewed from the incident light side (counterclockwise direction if it is observed from the outgoing beam side). The analyzer 2 has a polarization direction which is rotated at an angle of 45.degree. with respect to the polarizer 1 as indicated by an arrow y.fwdarw.z.
Only the polarized light rays among the light beam O from the light having a plane of polarization along an arrow Oy can pass through the polarizer 1, then the plane of polarization is rotated at an angle of 45.degree. by the action of the Farady rotator 3 and is in agreement with the polarization direction y.fwdarw.z of the analyzer 2. Therefore, the polarized light can pass through the analyzer 2. The majority of the transmitted light beam O (having a plane of polarization Oy.fwdarw.z) is made incident upon the subsequent optical system such as an optical fiber (not shown), while a part thereof is surface-reflected at the end face of the optical fiber. The reflected light rays R (having a plane of polarization Ry.fwdarw.z) pass through the analyzer 2 in the direction opposite to the foregoing incident light rays and the plane of polarization thereof is counterclockwise rotated at an angle of 45.degree. by the action of the Farady rotator 3. For this reason, the reflected light cannot pass through the polarizer 1 since the plane of polarization is perpendicular to that of the polarizer 1 and the polarizer thus has a light-extinction ability to thereby prevent the reflected light R from making noises.
There has been investigated wavelength multiplex communication in accordance with requirements for high speed transmission and large capacity communication in the recent optical communication systems. Optical isolators used therein must be effective for any wavelength ranges if light rays of a plurality of wavelength ranges pass through it. However, Farady rotators 3 used in the conventional optical isolators have wavelength-dependency in their Farady rotational angles. More specifically, the rotational angle of the plane of polarization thereof varies depending on the wavelengths of the transmitted light rays even if the optical path and the intensity of the magnetic field are maintained at constant levels. For this reason, the light-extinction ability of the optical isolator is greatly influenced by the wavelengths of the transmitted light and, therefore, it is difficult for a single optical isolator to deal with such wavelength multiplex communication.
To deal with wavelength multiplex communication by the use of a single optical isolator, Japanese Patent Provisional Publication Nos. 63-17426 and 63-49728 disclose multi-stage type optical isolators which comprise a plurality of Farady rotators whose rotational angles are different from one another depending on wavelengths. However, the optical isolators as disclosed in the foregoing patents are multi-stage type ones comprising a plurality of Farady rotators and correspondingly suffer from a problem that they have a high insertion loss. Moreover, this is retrograde to the recent requirement for miniaturization of optical communication systems.